The auto-battle feature allows you to have the game play itself in Story missions for you! Once this feature is unlocked in Chapter 2, you can use it across all game modes (Story, Portal, Hawkins National Lab, and Club Boss).
The auto-battle feature must be manually turned on with the start of each mission. To turn it on, tap the icon with two arrows in the top-right of the screen, just under the Flee button.
The auto-battle feature can be turned on and off as often as you want when in battle!